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        2020年12月16日 晚上
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            <h1 style="display: none">React不常用的生命周期</h1>
            
            <div class="markdown-body">
              <p>React16.3之后的生命周期：<br><img style="margin:0 auto;" src="/images/lifeCycle.png" srcset="/img/loading.gif" /><br>几个不常用的生命周期：</p>
<h4 id="getDerivedStateFromProps-props-state"><a href="#getDerivedStateFromProps-props-state" class="headerlink" title="getDerivedStateFromProps(props, state)"></a>getDerivedStateFromProps(props, state)</h4><blockquote>
<p>使用场景：在初始化组件数据时，我们想要将组件接收的参数props中的数据添加到组件内部中的state中去， 期望组件响应 props 的变化。</p>
</blockquote>
<p>执行过程：该回调会在render之前执行，返回一个对象来更新state，或者返回null，什么也不进行更新。</p>
<p>栗子：将props中的数据添加到state中去</p>
<div class="code-wrapper"><pre><code class="hljs javascript">state = &#123;
    <span class="hljs-attr">c</span>: <span class="hljs-number">1</span>,
    <span class="hljs-attr">t</span>: <span class="hljs-number">2</span>
&#125;

<span class="hljs-keyword">static</span> <span class="hljs-function"><span class="hljs-title">getDerivedStateFromProps</span>(<span class="hljs-params">props, state</span>)</span> &#123;
    <span class="hljs-built_in">console</span>.log(<span class="hljs-string">&#x27;变化了111111111&#x27;</span>, props, state);
    <span class="hljs-keyword">return</span> &#123;
        <span class="hljs-attr">c</span> : props.count
    &#125;
    <span class="hljs-comment">// return null 不更新任何内容</span>
&#125;</code></pre></div>
<h4 id="shouldComponentUpdate-nextProps-nextState"><a href="#shouldComponentUpdate-nextProps-nextState" class="headerlink" title="shouldComponentUpdate(nextProps, nextState)"></a>shouldComponentUpdate(nextProps, nextState)</h4><blockquote>
<p>使用场景：直接决定 React 的组件是否进行更新</p>
</blockquote>
<p>无论是与当前的 props 还是与当前的 state 进行比较，最终总是会得出一个 true 或者是 false，来决定组件是否更新(render)。</p>
<p>栗子：</p>
<div class="code-wrapper"><pre><code class="hljs javascript"><span class="hljs-function"><span class="hljs-title">shouldComponentUpdate</span>(<span class="hljs-params">nextProps, nextState</span>)</span> &#123;
    <span class="hljs-keyword">if</span>(....) &#123;
        <span class="hljs-keyword">return</span> <span class="hljs-literal">false</span>; <span class="hljs-comment">// 不render</span>
    &#125; <span class="hljs-keyword">else</span> &#123;
        <span class="hljs-keyword">return</span> <span class="hljs-literal">true</span> <span class="hljs-comment">// 要render</span>
    &#125;
&#125;</code></pre></div>

<h4 id="getSnapshotBeforeUpdate-prevProps-prevState"><a href="#getSnapshotBeforeUpdate-prevProps-prevState" class="headerlink" title="getSnapshotBeforeUpdate(prevProps, prevState)"></a>getSnapshotBeforeUpdate(prevProps, prevState)</h4><blockquote>
<p>使用场景: 这个方法能够使得组件可以在可能更改之前从 DOM 捕获一些信息，比如滚动的位置等等。</p>
</blockquote>
<p>注意，这个方法的返回值会作为第三个参数，传给<code>componentDidUpdate(nextPros, nextState, snapshot)</code>;<br>该方法必须返回一个值或者是返回null</p>
<p>栗子：</p>
<div class="code-wrapper"><pre><code class="hljs javascript"><span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">ScrollingList</span> <span class="hljs-keyword">extends</span> <span class="hljs-title">React</span>.<span class="hljs-title">Component</span> </span>&#123;
  <span class="hljs-function"><span class="hljs-title">constructor</span>(<span class="hljs-params">props</span>)</span> &#123;
    <span class="hljs-built_in">super</span>(props);
    <span class="hljs-built_in">this</span>.listRef = React.createRef();
  &#125;

  <span class="hljs-function"><span class="hljs-title">getSnapshotBeforeUpdate</span>(<span class="hljs-params">prevProps, prevState</span>)</span> &#123;
    <span class="hljs-comment">// 捕获滚动的位置，以便后面进行滚动 注意返回的值</span>
    <span class="hljs-keyword">if</span> (prevProps.list.length &lt; <span class="hljs-built_in">this</span>.props.list.length) &#123;
      <span class="hljs-keyword">const</span> list = <span class="hljs-built_in">this</span>.listRef.current;
      <span class="hljs-keyword">return</span> list.scrollHeight - list.scrollTop;
    &#125;
    <span class="hljs-keyword">return</span> <span class="hljs-literal">null</span>;
  &#125;

  <span class="hljs-function"><span class="hljs-title">componentDidUpdate</span>(<span class="hljs-params">prevProps, prevState, snapshot</span>)</span> &#123;
    <span class="hljs-comment">// 如果有 snapshot 会进行滚动的调整，这样子就不会立即将之前的内容直接弹上去</span>
    <span class="hljs-keyword">if</span> (snapshot !== <span class="hljs-literal">null</span>) &#123;
      <span class="hljs-keyword">const</span> list = <span class="hljs-built_in">this</span>.listRef.current;
      list.scrollTop = list.scrollHeight - snapshot;
    &#125;
  &#125;

  <span class="hljs-function"><span class="hljs-title">render</span>(<span class="hljs-params"></span>)</span> &#123;
    <span class="hljs-keyword">return</span> (
      <span class="xml"><span class="hljs-tag">&lt;<span class="hljs-name">div</span> <span class="hljs-attr">ref</span>=<span class="hljs-string">&#123;this.listRef&#125;</span>&gt;</span>&#123;/* ...contents... */&#125;<span class="hljs-tag">&lt;/<span class="hljs-name">div</span>&gt;</span></span>
    );
  &#125;
&#125;</code></pre></div>

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